414 research outputs found
On the Formation of Multiple-Shells Around Asymptotic Giant Branch Stars
Two types of models for the formation of semi-periodic concentric multiple
shells (M-shells) around asymptotic giant branch (AGB) stars and in planetary
nebulae are compared against observations. Models that attribute the M-shells
to processes in an extended wind acceleration zone around AGB stars result in
an optically thick acceleration zone, which reduces the acceleration efficiency
in outer parts of the extended acceleration zone. This makes such models an
unlikely explanation for the formation of M-shells. Models which attribute the
M-shell to semi-periodic variation in one or more stellar properties are most
compatible with observations. The only stellar variation models on time scales
of 50-1500 years that have been suggested are based on an assumed solar-like
magnetic cycle. Although ad-hoc, the magnetic cycle assumption fits naturally
into the increasingly popular view that magnetic activity plays a role in
shaping the wind from upper AGB stars.Comment: 8 pages, Submitted to Ap
The Shapes of AGB Envelopes as Probes of Binary Companions
We describe how the large scale geometry of the circumstellar envelopes of
asymptotic giant branch stars can be used to probe the presence of unseen
stellar companions. A nearby companion modifies the mass loss by
gravitationally focusing the wind towards the orbital plane, and thereby
determines the shape of the envelope at large distances from the star. Using
available simulations, we develop a prescription for the observed shapes of
envelopes in terms of the binary parameters, envelope orientation, and type of
observation. The prescription provides a tool for the analysis of envelope
images at optical, infrared, and millimetre wavelengths, which can be used to
constrain the presence of companions in well observed cases. We illustrate this
approach by examining the possible role of binary companions in triggering the
onset of axi-symmetry in planetary nebula formation. If interaction with the
primary leads to axi-symmetry, the spherical halos widely seen around newly
formed nebulae set limits on the companion mass. Only low mass objects may
orbit close to the primary without observable shaping effects: they remain
invisible until the interaction causes a sudden change in the mass loss
geometry.Comment: 11 pages, 7 figures, to appear in MNRA
Solar-Like Cycle in Asymptotic Giant Branch Stars
I propose that the mechanism behind the formation of concentric semi-periodic
shells found in several planetary nebulae (PNs) and proto-PNs, and around one
asymptotic giant branch (AGB) star, is a solar-like magnetic activity cycle in
the progenitor AGB stars. The time intervals between consecutive ejection
events is about 200-1,000 years, which is assumed to be the cycle period (the
full magnetic cycle can be twice as long, as is the 22-year period in the sun).
The magnetic field has no dynamical effects; it regulates the mass loss rate by
the formation of magnetic cool spots. The enhanced magnetic activity at the
cycle maximum results in more magnetic cool spots, which facilitate the
formation of dust, hence increasing the mass loss rate. The strong magnetic
activity implies that the AGB star is spun up by a companion, via a tidal or
common envelope interaction. The strong interaction with a stellar companion
explains the observations that the concentric semi-periodic shells are found
mainly in bipolar PNs.Comment: 10 pages, submitted to Ap
Model for the hydrogen adsorption on carbon nanostructures
The hydrogen sorption capacity of carbon nanostructures was for several years a very controversial subject. Theoretical models have been published demonstrating a great potential for a large hydrogen sorption capacity of carbon nanostructures. Here we present a simple empirical model where condensation of hydrogen as a monolayer at the surface of nanotubes as well as bulk condensation in the cavity of the tube is assumed. The maximum potential amount of hydrogen absorbed according to the model was calculated to be 2.28×10-3mass % S[m2g-1]=3.0mass % for the adsorption of a monolayer hydrogen at the surface. The condensation of hydrogen in the cavity of the tube leads to a potential absorption for single wall nanotubes starting at 1.5mass % and increasing with the diameter of the tubes. The experimentally measured hydrogen capacity of the nanotube samples correlates with the B.E.T. specific surface area. The slope of the linear relationship is 1.5×10-3mass %/m2g-1. Therefore, the extrapolated maximum discharge capacity of a carbon sample is 2mass %. Furthermore, it can be concluded, that the hydrogen sorption mechanism is related to the surface of the sample, i.e. a surface adsorption proces
The low wind expansion velocity of metal-poor carbon stars in the Halo and the Sagittarius stream
We report the detection, from observations using the James Clerk Maxwell
Telescope, of CO J 3 2 transition lines in six carbon stars, selected
as members of the Galactic Halo and having similar infrared colors. Just one
Halo star had been detected in CO before this work. Infrared observations show
that these stars are red (J-K 3), due to the presence of large dusty
circumstellar envelopes. Radiative transfer models indicates that these stars
are losing mass with rather large dust mass-loss rates in the range 1--3.3
Myr, similar to what can be observed in the
Galactic disc. We show that two of these stars are effectively in the Halo, one
is likely linked to the stream of the Sagittarius Dwarf Spheroidal galaxy (Sgr
dSph), and the other three stars certainly belong to the thick disc. The wind
expansion velocities of the observed stars are low compared to carbon stars in
the thin disc and are lower for the stars in the Halo and the Sgr dSph stream
than in the thick disc. We discuss the possibility that the low expansion
velocities result from the low metallicity of the Halo carbon stars. This
implies that metal-poor carbon stars lose mass at a rate similar to metal-rich
carbon stars, but with lower expansion velocities, as predicted by recent
theoretical models. This result implies that the current estimates of mass-loss
rates from carbon stars in Local Group galaxies will have to be reconsidered.Comment: 10 pages, 7 figures, accepted for publication in MNRA
Optical Signatures of Circumstellar Interaction in Type IIP Supernovae
We propose new diagnostics for circumstellar interaction in Type IIP
supernovae by the detection of high velocity (HV) absorption features in Halpha
and He I 10830 A lines during the photospheric stage. To demonstrate the
method, we compute the ionization and excitation of H and He in supernova
ejecta taking into account time-dependent effects and X-ray irradiation. We
find that the interaction with a typical red supergiant wind should result in
the enhanced excitation of the outer layers of unshocked ejecta and the
emergence of corresponding HV absorption, i.e. a depression in the blue
absorption wing of Halpha and a pronounced absorption of He I 10830 A at a
radial velocity of about -10,000 km/s. We identify HV absorption in Halpha and
He I 10830 A lines of SN 1999em and in Halpha of SN 2004dj as being due to this
effect. The derived mass loss rate is close to 10^{-6} Msun/yr for both
supernovae, assuming a wind velocity 10 km/s. We argue that, in addition to the
HV absorption formed in the unshocked ejecta, spectra of SN 2004dj and SN
1999em show a HV notch feature that is formed in the cool dense shell (CDS)
modified by the Rayleigh-Taylor instability. The CDS results from both shock
breakout and radiative cooling of gas that has passed through the reverse shock
wave. The notch becomes dominant in the HV absorption during the late
photospheric phase, ~60 d. The wind density deduced from the velocity of the
CDS is consistent with the wind density found from the HV absorption produced
by unshocked ejecta.Comment: 38 pages, 12 figures, ApJ, in pres
Herschel's view into Mira's head
Herschel's PACS instrument observed the environment of the binary system Mira
Ceti in the 70 and 160 micron bands. These images reveal bright structures
shaped as five broken arcs and fainter filaments in the ejected material of
Mira's primary star. The overall shape of the IR emission around Mira deviates
significantly from the expected alignment with Mira's exceptionally high space
velocity. The observed broken arcs are neither connected to each other nor are
they of a circular shape; they stretch over angular ranges of 80 to 100
degrees. By comparing Herschel and GALEX data, we found evidence for the
disruption of the IR arcs by the fast outflow visible in both Halpha and the
far UV. Radial intensity profiles are derived, which place the arcs at
distances of 6-85" (550 - 8000 AU) from the binary. Mira's IR environment
appears to be shaped by the complex interaction of Mira's wind with its
companion, the bipolar jet, and the ISM.Comment: 4 page
New, nearby bright southern ultracool dwarfs
We report the discovery of twenty-one hitherto unknown bright southern
ultracool dwarfs with spectral types in the range M7 to L5.5, together with new
observations of a further three late M dwarfs previously confirmed. Three more
objects are already identified in the literature as high proper motion stars;we
derive their spectral types for the first time. All objects were selected from
the 2MASS All Sky and SuperCOSMOS point source databases on the basis of their
optical/near-infrared colours, -band magnitudes and proper motions. Low
resolution (R 1000) spectroscopy with the ESO/NTT SOFI spectrograph
has confirmed the ultracool nature of 24 targets, out of a total of 25
candidates observed. Spectral types are derived by direct comparison with
template objects and compared to results from HO and FeH indices. We also
report the discovery of one binary, as revealed by SOFI acquisition imaging;
spectra were taken for both components. The spectral types of the two
components are L2 and L4 and the distance 19 pc. Spectroscopic distances
and transverse velocities are derived for the sample. Two L5 objects lie
only 10 pc distant. Such nearby objects are excellent targets for
further study to derive their parallaxes and to search for fainter, later
companions with AO and/or methane imaging.Comment: 11 pages, 10 figures, accepted to MNRA
On the reliability of mass-loss-rate estimates for AGB stars
In the recent literature there has been some doubt as to the reliability of
CO multi-transitional line observations as a mass-loss-rate estimator for AGB
stars. Mass-loss rates for 10 intermediate- to high-mass-loss-rate AGB stars
are derived using a detailed non-LTE, non-local radiative transfer code based
on the Monte-Carlo method to model the CO radio line intensities. The
circumstellar envelopes are assumed to be spherically symmetric and formed by
constant mass-loss rates. The energy balance is solved self-consistently and
the effects of dust on the radiation field and thermal balance are included. An
independent estimate of the mass-loss rate is also obtained from the
combination of dust radiative transfer modelling with a dynamical model of the
gas and dust particles. We find that the CO radio line intensities and shapes
are successfully reproduced for the majority of our objects assuming a constant
mass-loss rate. Moreover, the CO line intensities are only weakly dependent on
the adopted micro-turbulent velocity, in contrast to recent claims in the
literature. The two methods used in the present work to derive mass-loss-rates
are consistent within a factor of ~3 for intermediate- to high-mass-loss-rate
objects, indicating that this is a lower limit to the uncertainty in present
mass-loss-rate estimates. We find a tentative trend with chemistry. Mass-loss
rates from the dust/dynamical model are systematically higher than those from
the CO model for the carbon stars and vice versa for the M-type stars. This
could be ascribed to a discrepancy in the adopted CO/H_2-abundance ratio, but
we caution that the sample is small and systematic errors cannot be excluded.Comment: 18 pages, 17 figures, accepted for publication in A&
Water vapor emission from IRC+10216 and other carbon-rich stars: model predictions and prospects for multitransition observations
We have modeled the emission of H2O rotational lines from the extreme C-rich
star IRC+10216. Our treatment of the excitation of H2O emissions takes into
account the excitation of H2O both through collisions, and through the pumping
of the nu2 and nu3 vibrational states by dust emission and subsequent decay to
the ground state. Regardless of the spatial distribution of the water
molecules, the H2O 1_{10}-1_{01} line at 557 GHz observed by the Submillimeter
Wave Astronomy Satellite (SWAS) is found to be pumped primarily through the
absorption of dust-emitted photons at 6 m in the nu2 band. As noted by
previous authors, the inclusion of radiative pumping lowers the ortho-H2O
abundance required to account for the 557 GHz emission, which is found to be
(0.5-1)x10^{-7} if the presence of H2O is a consequence of vaporization of
orbiting comets or Fischer-Tropsch catalysis. Predictions for other
submillimeter H2O lines that can be observed by the Herschel Space Observatory
(HSO) are reported. Multitransition HSO observations promise to reveal the
spatial distribution of the circumstellar water vapor, discriminating among the
several hypotheses that have been proposed for the origin of the H2O vapor in
the envelope of IRC+10216. We also show that, for observations with HSO, the
H2O 1_{10}-1_{01} 557 GHz line affords the greatest sensitivity in searching
for H2O in other C-rich AGB stars.Comment: 35 pages, 12 figures, to be published in The Astrophysical Journa
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